Today, many sensors such as MEMS accelerometers, gyroscopes and microphones include a capacitive interface circuit. In capacitive sensor readout circuits, full or half capacitive bridges can be formed by sensing capacitors of a sensing element. The capacitive bridges generate a capacitance change in response to a stimulus to be sensed. The capacitance change is converted to a voltage by the readout interface circuit, which is normally implemented as a charge amplifier or voltage amplifier.
Most MEMS capacitive sensors, particularly monolithic MEMS capacitive sensors, have very small sensing capacitance and very low transducer sensitivity. The sensed signals in these sensors typically have a bandwidth from DC to a few kilohertz (kHz). For example, monolithic surface micromachined accelerometers and gyroscopes have a sensing capacitance well below one picofarad (1 pF) and a sensed voltage signal at the sensing element that is in the microvolt (μV) range or even lower. Large signal gain must be provided by the readout circuits in order to achieve a useful overall sensitivity. Designing readout circuits with high dynamic range and low power dissipation for these capacitive sensors may be challenging since the small value of the sensing capacitance results in high output impedance and the small signals are greatly affected by parasitic capacitances and other non-idealities in the readout circuit. The 1/f noise is a major noise source in the signal frequency range and remains significant even when the electronic noise is minimized for small parasitic capacitance. In sub-micron complementary metal oxide semiconductor (CMOS) technology, a small sensing capacitance of less than 1 pF results in a 1/f noise corner of the readout circuit around one megahertz (1 MHz) or even higher.
To effectively remove the 1/f noise, correlated-double sampling is widely used in switched-capacitor (SC) charge amplifier readout circuits. Additionally, chopper amplifier technique is mostly used in continuous-time voltage (CTV) sensing circuits. By avoiding the kT/C noise and the noise folding effect in SC circuits, CTV circuits typically achieve lower noise than their SC counterparts as long as the chopping clock frequency is chosen at about the flicker noise corner frequency or higher.
In SC and conventional chopper-amplifier based CTV readout circuits, high clock frequency is needed to effectively remove 1/f noise. In addition, low transducer sensitivity generally requires that a large signal gain must be provided by the readout circuits. Together with the high clock frequency, this low sensitivity inevitably results in a high-gain bandwidth requirement of amplifiers in the readout circuits. This severely limits the minimization of the power dissipation of the readout circuits. Most currently available monolithic capacitive sensors with high resolution or high dynamic range normally dissipate a power well above a few milli watts (mWs) or even tens of mWs. This large power level greatly limits their application in portable consumer electronics market.